Abstract
To elucidate the migration mechanism of a muscle attachment during growth, the attachment of the deep layer of the masseter muscle to rat mandibles aged 50 to 80 days was examined by light microscopy, polarized light microscopy and lead-labelling technique (Okada & Mimura, 1938).
The deep layer of the masseter muscle was divided into three attachments, according to the arrangement of attached fibers at the interface between bone and muscle and the ossification type at the bone surface.
In the periosteal attachment, endomysium of muscle fibers attached solely to the fibrous layer of the periosteum and immature Sharpey's fibers extended occasionally from the periosteal fibrous layer to the bone surface which revealed lamellar periosteal ossification. In the tendinous attachment, the tendon attached to the mandible through the fibrocartilage tissue which consisted of the plexus of immature fibers and the active fibrocartilage cells, and serrate fibrocartilaginous calcification was observed at the bone surface. In the transitional zone, the mode of attachment was a transitional type of both the periosteal and the tendinous attachments, and serrate periosteal ossification was observed.
It was concluded that the mode of attachment changing from the tendinous attachment through the transitional zone to the periosteal attachment may be caused by rearrangement of the attached fibers during growth and the mode of ossification changed from fibrocartilaginous calcification to periosteal ossification. These results suggested that the attachment of the deep layer of the masseter muscle to the rat mandible migrated, maintaining the relative position to the mandible.
